1. Field of the Invention
The present invention relates to inverters for converting DC to alternating current with a neutral.
2. Description of the Prior Art
An article entitled "The Resonant DC Link Converter - A New Concept in Static Power Conversion", D. M. DIVAN CH2272-3/86/0000-0648 1986 IEEE discloses a power converter for use with a balanced load such as a motor. Three phase AC at a fundamental frequency without a neutral is generated by selectively switching switches in a switching bridge to create an envelope, varying at the fundamental frequency and containing a resonant frequency higher than the fundamental frequency produced by a parallel resonant circuit, which is applied to the load. Switching losses in the switches are minimized by clamping the DC bus to zero while changing the conductivity of the switches. See column 1, page 650, under the heading "Basic Principles of the Resonant D Link Converter".
An article entitled "A DC-AC Power Conversion Technique Using Twin Resonant High Frequency Links", V. T. Ranganathan, Phoivos D. Ziogas and Victor P. Stefanovic, 0093-9994/83/0500-0393 1983 IEEE discloses the generation of a low frequency by the generation of difference signal from two high frequency sources having frequencies fc+fo and fc-fo. The difference signal is applied to the load by means of selectively switching a switching bridge connected to the electrical load.
A power supply has been developed by Westinghouse Electric Corporation having a switching matrix which chops a DC potential coupled to the switching matrix to create a staircase waveform at a desired fundamental frequency to produce three phase AC. With reference to FIG. 2 of the present invention, the prior art Westinghouse topology has a switching matrix containing switches Q.sub.1, Q.sub.1, Q.sub.2, Q.sub.2, Q.sub.3 and Q.sub.3 and the elements to the right thereof. Unlike the present invention, the potential switched by the switching matrix is DC. Furthermore, as far is known, the pairs of switches Q.sub.1 and Q.sub.1, Q.sub.2 and Q.sub.2 and Q.sub.3 and Q.sub.3 are not short circuited at any point in time as is the case with the present invention as a consequence of the pulses in the last 180.degree. of waveform Q.sub.1 of FIG. 3A and the pulses Q.sub.1 in the first 180.degree. of the waveform Q.sub.1 of FIG. 3B. For an unbalanced load, current flows through the load, the neutral, and back to the negative terminal of the DC source.
Inverters are known which convert a DC potential to a single phase and other inverters are known which convert a DC potential to multiple phases. These single phase and multiple phase inverters work on the principal that the DC power source is chopped and filtered to produce a single or three phase output with a neutral.
It is known that switching losses from a transistor may be minimized by switching the transistor state during the time that the voltage across the transistor is zero. See U.S. Pat. Nos. 4,138,715, 4,196,469 and 4,413,313.
Alternating current power supplies are known which utilize a LC resonating circuit for providing a fundamental frequency which have energy fed into the resonating circuit to replace energy drawn off by a load. See U.S. Pat. Nos. 4,196,469 and 4,574,342.